How do you blow a conch?

It's done just like you do to any other horn instrument you make the sound with your lips and you blow into the shell

First you generally have to cut off the end so there is a small hole.  Like this:  http://www.myriahsbazaar.com/v/vspfiles/photos/R33...  You can buy them like this, easier than cutting it.

Then you blow it like a trumpet.  This takes practice.  Hold your lips tight and close together stiffly and blow so that it makes a squeaking sound.  Do this at the hole end of the conch and it will get louder.  If you are good at it, you can probably get two or three different notes.  

If you know anybody who plays the trumpet or bugle, they can show you.  It is the same technique.  Or maybe find somebody on YouTube.